Many portable devices, for example mobile telephones, contain a number of acoustic transducers, such as microphones, earpieces and speakers. Such transducers are key components in mobile phone audio/acoustic design. Generally, there will be one or more sound channels or back cavities associated with each acoustic transducer. Such sound channels can ensure a certain frequency response is obtained for the transducer, and must be carefully designed as part of the mechanical configuration of the device hardware. Small changes in the size and configuration of the sound channels or cavities can have a large effect on the acoustic properties of the combined transducer/sound channel.
In known acoustic transducer configurations, the mechanical design of the sound channels is fixed at the point of hardware design and manufacture of the device is completed, and cannot be later adapted during use for a specific purpose or desired configuration. Instead, the desired acoustic properties are produced by filtering the electrical signal representing the sound output before the signal is applied to the transducer. Typically, this requires the use of significant processing power, commonly provided by dedicated digital signal processors (DSPs).
Furthermore there is a limit to the modification of the acoustic response of the transducer which can be carried out in the DSP.
Microphones are typically designed to be as sensitive as possible so that the signal to noise ratio is as high as possible. The consequences of the design to be as sensitive as possible are that the gap between the membrane and the back plate typically must be as small as possible in order to maximise the capacitance between the two plates (the membrane being the first plate, and the back plate being the second plate). Furthermore to design the microphone to be as sensitive as possible, the compliancy of the membrane should be as high as possible so that the membrane vibrates as sensitively as possible along with any sound pressure level change.
The problem associated with such a design is that the membrane of the microphone can touch the back plate easily, for example when a large sound pressure level is experienced. This touching or contact could cause the membrane to stick to the back plate permanently, in other words producing a permanent malfunction of the microphone. When the membrane sticks or touches to the back plate temporarily, this produces a temporary malfunction whereby the microphone is non-functional until it can be reset. Furthermore if the membrane only touches the back plate temporarily and does not stick to the back plate, the resultant signal output by the microphone produces a bad audible distortion. This audible distortion is often called microphone saturation and cannot easily be remedied or compensated for using digital signal processing.
An example of the limitations of the mechanical design of typical microphone transducers is that of wind noise. Wind noise is a problem particularly for miniaturised designs such as found in mobile phone where there is no room for mechanical protection of the microphone from wind such as used in broadcast microphones like wind screens or foam protectors. Furthermore filtering out the wind noise from the signal in the electrical domain, not only requires significant processing power in a digital signal processor, but typically produces poor results as the sound pressure levels generated by the wind cause the microphone acoustic element to saturate.
Thus when the microphone is exposed to significant wind the microphone plates are forced together and produces a saturated signal outputting “wind noise” which cannot be removed from the signal.
A further example of the limitations of the mechanical design of a typical microphone would be at a loud event, such as a rock concert. In such events, the optimal sensitivity of the microphone is significantly less than the optimal sensitivity in quiet surroundings. Too high a sensitivity of the microphone during such events will cause the microphone to saturate at the high sound pressure levels and the resulting audio signal is heavily distorted and compressed. The results of which is a big drop in quality and a barely listenable recording of the event.
Although the sensitivity and mechanical saturation suppression can be affected by choosing the design of the microphone to have the desired mechanical or acoustical properties, these are typically fixed in manufacturing which requires compromises to be made in the design and during the use of the component. Furthermore as discussed, although there are ways to adjust the sensitivity of microphones such as adjusting the gain in the microphone preamplifier, or by changing the bias voltage of the microphone element, these techniques cannot overcome the problem of mechanical saturation of the microphone in loud or windy conditions.